https://leetcode.cn/problems/symmetric-tree/

bool _isSymmetric(struct TreeNode* left, struct TreeNode* right)
{
    if (left == NULL && right == NULL)
    {
        return true;
    }
    else if (left == NULL || right == NULL)
    {
        return false;
    }
    else
    {
        if (left->val != right->val)
        {
            return false;
        }
        else {
            return _isSymmetric(left->left, right->right) &&
                _isSymmetric(left->right, right->left)
        }
    }
}
bool isSymmetric(struct TreeNode* root) {
    return  _isSymmetric(root->left, root->right);

}